Long Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, as shown in FIG. 2. Each radio frame consists of ten equally-sized subframes of length Tsubframe=1 ms.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RB), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of virtual resource blocks (VRB) and physical resource blocks (PRB) has been introduced in LTE. The actual resource allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain, thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols (CRS), which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
The LTE Rel-10 specifications have recently been standardized, supporting Component Carrier (CC) bandwidths up to 20 MHz, which is the maximal LTE Rel-8 carrier bandwidth. Hence, an LTE Rel-10 operation wider than 20 MHz is possible and appear as a number of LTE carriers to an LTE Rel-10 terminal.
In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals (i.e., non-legacy terminals) compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CC, where the CC have, or at least has the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 4.
The Rel-10 standard supports up to 5 aggregated carriers where each carrier is limited in the RF specifications to have one of six bandwidths namely 6, 15, 25, 50, 75 or 100 RB, corresponding to 1.4, 3 5 10 15 and 20 MHz respectively.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in the network may be different from the number of CCs seen by a terminal: A terminal may for example support more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs.
During initial access a LTE Rel-10 terminal behaves similar to a LTE Rel-8 terminal. Upon successful connection to the network a terminal may—depending on its own capabilities and the network—be configured with additional CCs in the UL and DL. Configuration is based on Radio Resource Control (RRC). Due to the heavy signaling and rather slow speed of RRC signaling it is envisioned that a terminal may be configured with multiple CCs even though not all of them are currently used. If a terminal is activated on multiple CCs this would imply it has to monitor all downlink (DL) CCs for Physical Downlink Control Channel (PDCCH) and Physical Downlink Shared Channel (PDSCH). This implies a wider receiver bandwidth, higher sampling rates, etc. resulting in high power consumption.
Two types of carriers are referred to herein. The first type of carrier is a Rel-8 backward compatible carrier. It is characterized by that Rel-8, Rel-9 and Rel-10 User Equipment (UE) can operate on it. For simplicity it is referred to as carrier type A.
The second carrier type is described as a carrier type that contains either no CRS at all or much less CRS either in frequency, by for example a reduction of the bandwidth the CRS covers to be smaller than the carrier bandwidth, or in time, by for example not transmitting any CRS in some pre-defined subframes, or in both frequency and time compared to a type A carrier. It further may not contain any PDCCH but only the enhanced Control Channels eCCH including the enhanced PDCCH (ePDCCH) and/or the enhanced Physical Hybrid Automatic Retransmission reQuest (HARQ) Indicator Channel (ePHICH), which do not rely on CRS for demodulation. This type of carrier is referred to as carrier type B. Carrier type B is attractive for its energy efficiency properties, its low control and reference signal overhead and low level of interference generation in networks when compared to carrier type A.
The lack of CRS and/or PDCCH, PHICH, PCFICH will make this type of carrier, i.e., carrier type B, not accessible by legacy release UEs when deployed, i.e. it is not backwards compatible. Carrier type A and carrier type B are illustrated in FIG. 5 and FIG. 6 respectively.
The definition of the fields used in FIG. 5 are shown in FIG. 5 only and will be used also in relation to other figures herein even if left out from those figures. More specifically, FIG. 5 shows carrier type A in time (along the horizontal axis) and in frequency (along the vertical axis). FIG. 5 shows the carrier structured in time as 5 different subrames. In general, the Physical Control Format Indicator Channel—Physical Downlink Control Channel/Physical Hybrid ARQ Indicator Channel (PCFICH/PDCCH/PHICH) occupies the beginning of each subframe across the carrier bandwidth, and the PDSCH (including the ePDCCH) occupies the rest of each subframe. That said, the CRS is transmitted in a pattern across the carrier, and in FIG. 5 shows up as “dots” distributed over the figure. By contrast, FIG. 6 shows that carrier type B does not transmit the PCFICH/PDCCH/PHICH, and only transmits CRS within the second subframe and across a portion of the carrier bandwidth (outlined as a rectangle within the second subframe). In both FIGS. 5 and 6, the Physical Broadcast Channel/Primary Synchronization Signal/Secondary Synchronization Signal (PBCH/PSS/SSS) is transmitted within this rectangle, after the PCFICH/PDCCH/PHICH (in FIG. 5).
Carrier type B can only be accessible by terminals of the new release and not of legacy releases as it is non-backward compatible. At the time when carrier type B will be deployed in networks there will only be limited set of such new release terminals available that has the capability to access it and receive data on it. At the same time there will be a large population of legacy release terminals operating in existing networks, i.e. terminals only capable of accessing carriers of type A.